Method of producing fine grained steel



Oct. 11, 1966 App/ax. /00F 0 L 0 J t D R Y Q: 4 1 E k k 5 b k Be low w Transformation Range Room Tempe/a lure ch W or R. A. GRANGE ETAL METHOD OF PRODUCING FINE GRAINED STEEL Filed May 28, 1963 'Ff'a E lNl/ENTORS RAYMOND A. GRANGE and RONALD 5. MULHAUSE'R Aflomey United States Patent METHUD 0F PRODUCING FINE GRAINED STEEL Raymond A. Grange, Washington Township, Westmoreland County, and Ronald S. Mulhauser, Jelferson Borough, Pa., assignors to United States Steel Corporation,

a corporation of Delaware Filed May 28, 1963, Ser. No. 283,887 7 Claims. (Cl. 148-12.4)

This invention involves the production of fine austenite grain size in steels and more particularly the production of ultrafine austenite grain size in steels hardenable by thermomechanical treatment.

It is well known that the mechanical properties and resistance to cracking and distortion incident to hardening steels progressively improve with a decrease in austenite grain size. Thus it is desirable to produce commercially an ultrafine austenite grain size in steels hardenable by heat treatment, i.e., a grain size no larger than #11 ASTM and preferably a grain size of #13 ASTM or smaller.

It is accordingly an object of the present invention to provide an ultrafine austenite grain size in steels harden- :able by heat treatment.

It is a further object to provide an ultrafine austenite grain size in steels hardenable by heat treatment in an economical and efficient manner.

The foregoing and further objects will be apparent from the following specification when read in conjunction with the attached drawing wherein:

FIGURE 1 is a schematic representation of the cyclic treatment of this invention; and

FIGURE 2 is a graphic representation [of the heating and cooling involved in the cyclic treatment.

We have discovered that an ultrafine austenite grain size can be produced in a reliable and efficient manner by repeatedly rapidly heating a workpiece composed of steel hardenable by heat treatment, working and cooling it in a controlled manner. In accordance with our invention, steel of the type that is hardenable by heat treatment is treated cyclically by rapidly heating it to just above its AC3 temperature and below its grain coarsening temper-ature, thereafter while at such temperature rapidly deforming the workpiece at least sufficiently to deform the steel grains throughout the workpiece and then rapidly cooling the workpiece by air cooling or quenching to below the transformation range of the steel. The rapid heating, working and cooling is repeated sufficient times to produce the fineness of grain desired and following the last heating and working, the workpiece is cooled at a rate which develops the desired microstructure in the product.

A marked refinement of austenite grain size occurs when many varieties of steel hardenable by heat treatment are heated rapidly to a temperature barely suflicient toaustentize the steel and then cooled moderately fast to below the transformation temperature range. The rapid heating must be very closely controlled to achieve the desired grain refinement. When heating is done in the conventional way in an air or gas atmosphere furnace, the heating rate is too slow and the relative rates of nucleation and growth of austenite are such that when the initial ferrite and carbide phases are converted to austenite the grain size of the latter is not particularly small. In conventional rapid heating by electrical resistance, induction, or liquidbath heating, the heating rate is sufficiently fast but is not effective for the purposes of this invention unless the heating is stopped within closely controlled limits. We have further discovered that deforming steel at temperatures close to the lower limit of the stable austenite range enhances grain refinement on subsequent rapid heating and that these two grain-refining processes can be combined to achieve an ultarfine austenite grain size.

The figures of the drawing schematically represent this 3,278,345 Patented Oct. 11, 1966 new grain-refining process. The steep strip S, initially at ambient temperature, is heated rapidly to a temperature T approximately F. above its A0 temperature by heaters 2. While electrical induction heaters are shown, electrical resistance or other rapid heating devices may be used. The exact optimum heating temperature will vary somewhat among different types of steel in accordance with the relative ease with which carbides dissolve in austenite. The aim of the rapid heating cycle is to heat only high enough and long enough to convert the initial ferrite-carbide aggregate to austenite. Suitable rapid heating results from electrical induction or resistance heating or by immersing the steel in a liquid lead or salt bath. Following the rapid heating, the steel is immediately de formed. FIGURE 1 illustrates deformation by rolls 4 but other types :of deformation, such as forging or extrusion, may be used. The principal requirement of the deformation is that it be done rapidly to minimize grain growth that occurs progressivelly with time at temperature and that the grains throughout the workpiece be deformed. Generally, such grain refinement results from reductions of 25% or more in thickness. After deformation, the steel is cooled to below its transformation temperature range at a rate sufficiently rapid to produce a microstructure of the class consisting of bainite or martensite or mixtures thereof. Air cooling is satisfactory for many materials less than about 0.25 inch thick, but larger sections may require air-blast cooling or quenching. The purpose of moderately rapid cooling is two-fold. First, to minimize grain growth caused by too long exposure to temperatures above its A0 and, second, to avoid a coarse transformation product such as ferrite-pearlite.

At this point, the steel has reduced grain size, but to obtain the desired ultrafine grain size, it is necessary to repeat the process several times. FIGURE 1 illustrates a 3-cycle process. The optimum number of cycles will depend upon the amount of reduction in each cycle and upon the type of steel being processed.

To illustrate the teachings of this invention, samples of strip of AISI 4340 steel were processed by the process of this invention and by conventional treatment as follows. The actual composition of the steel was Percent Carbon .42 Manganese .74 Silicon .30 Nickel 1.89 Chromium 0.76 Molybdenum 0.28

balance inon and residual impurities. The Ac temperature of this steel is 1400 F. and its transformation range is 1300 to 400 F.

In accordance with this invention, Specimen A of such steel was treated by this invention as follows:

(1) 0.200-inch-thick blank heated in lead at 1550 F. for '1 minute, reduced 25% in one pass and oil quenched; (2) Reheated in lead at 1550 F. for 20 seconds and reduced 33 in one pass and oil quenched;

(3) Reheated in lead at 1550 F. for 20 seconds and reduced 50% in one pass and oil quenched;

(4) Reheated in lead at 1550 F. for 8 seconds and oil quenched;

(5 Tempered at 450 'F. for 2 hours.

Specimen B of the same steel was treated by conventional processing as follows:

(1) 0.200-inch-thick blank hot rolled in conventional way to 0.050-inch-thick;

(2) Heated at 1600 F. for 1 hour and oil quenched;

(3) Tempered at 450 F. for 2 hours.

The comparative results of such treatments are shown in the following Table I:

Table I Austenite 0.2%

Grain Tensile Ofiset Elong. Specimen Structure Size, Strength, Yield in 1",

ASTM 1,000 Strength, per- No. p.s.i. 1,000 cent p.s.i.

A 450 F. Tempered 16 298. 7 276. 2 6

Martensite. B do 11 268. 4 231. 6 6

Specimens of strip of AISI 8650 steel having the following oomposition were given similar comparative treatments:

Percent Carbon 0.50 Manganese 0.77 Silicon 0.22 Nickel 0.60 Chromium 0.51 Molybdenum 0.22

balance iron and residual impurities. The Ae temperature of this steel is 1400 F. and the transformation range is 1325 to 400 F.

Specimen C was treated by this invention as follows:

(1) 0.200-inch-thick blank heated in lead at 1525 F. for 1 minute, reduced 25% in one pass and oil quenched; (2) Reheated in lead at 1525 F. for 30 seconds, re-

duced 33% in one pass and oil quenched;

(3) Reheated in lead at 1525 F. for 20 seconds, reduced 50% in one pass and oil quenched;

(4) Tempered at 400 F. for 2 hours.

Specimen D was given the following conventional treatment:

(1) 0.200-inch-thick blank hot rolled in conventional way to 0.050-inch-thick;

(2) Heated at 1600 F. for 1 hour and oil quenched;

(3) Tempered at 400 F. for 2 hours.

Comparative data obtained from these specimens is given in the following Table II:

In both of the above tables, the specimen processed according to the teachings of this invention has an ultrafine austenite grain size and higher strength without lower ductility as compared to the conventionally heat-treated, fine-grained specimen. The structure of all specimens is low temperature tempered martensite and these specimens are of the so-called ultra-high strength class. It should be realized, however, that the advantages of ultrafine grain size developed by our method are equally characteristic of steel treated to lower strength whether the microstructure is tempered martensite or some other transformation product other than ferrite-pearlite.

While we have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A method of producing ultrafine grain size in steels hardenable by thermomechanical treatment comprising rapidly heating the steel to slightly above its A0 temperature and before any substantial grain growth occurs, reducing the cross sectional area thereof sufficiently to deform substantially all of the grains thereof, rapidly cooling to below the transformation range of the steel and repeating such heating, repeating said working and cooling a plurality of times to produce an ultrafine grain size and following the last working, quickly cooling to substantially room temperature.

2. A method of producing ultrafine grain size in steels hardenable by thermomechanical treatment comprising rapidly heating the steel to about F. above its Ac temperature and before any substantial grain growth occurs, reducing the cross-sectional area thereof sufiiciently to deform substantially all of the grains thereof, rapidly cooling to below the transformation range of the steel to develop a martensitic, bainitic or bainitic-martensitic and repeating such heating, working and cooling a plurality of times to produce an ultrafine grain size of ASTM #13 or finer and following the last working, quickly cooling to substantially room temperature.

3. A method of producing ultrafine grain size in steels hardenable by thermomechanical treatment comprising rapidly heating the steel to slightly above its A0 temperature and before any substantial grain growth occurs, reducing the cross-sectional area thereof sufficiently to deform substantially all of the grains thereof, rapidly cooling to below the transformation range of the steel and repeating such heating, repeating said working and cooling a plurality of times to produce an ultrafine grain size in the steel and following the last working, quickly cooling to below the transformation range of the steel.

4. A method as in claim 3 wherein after a reduction of cross-sectional area prior to the last such reduction the steel is quenched to a martensitic microstructure.

5. A method as in claim 3 wherein after a reduction of cross-sectional area prior to the last such reduction the steel is quenched to a bainitic microstructure.

6. A method as in claim 3 wherein after a reduction of cross-sectional area prior to the last such reduction the steel is quenched to a microstructure consisting essentially of a mixture of martensite and bainite.

7. A method of producing ultrafine grain size in steels hardenable by thermomechanical treatment comprising rapidly heating the steel to about 100 F. above its AC3 temperature and before any substantial grain growth occurs, reducing the cross-sectional area thereof sufficiently to deform substantially all of the grains thereof, rapidly cooling to below the transformation range of the steel and repeating such heating, working and cooling a plurality of times to produce an ultrafine grain size of at least ASTM 13 in the steel and following the last working, quickly cooling to below the transformation range of the steel at a rate sufficiently rapid to obtain a microstructure other than ferrite-pearlite.

References Cited by the Examiner UNITED STATES PATENTS 2,791,500 5/1957 Foley et a1. 148-143 2,863,763 12/1958 Rosenberg et al 148-143 FOREIGN PATENTS 659,818 3/1963 Canada.

DAVID L. RECK, Primary Examiner. H. F. SAITO, Assistant Examiner. 

1. A METHOD OF PRODUCING ULTRAFINE GRAIN SIZE IN STEELS HARDENABLE BY THERMOMECHANICAL TREATMENT COMPRISING RAPIDLY HEATING THE STEEL TO SLIGHTLY ABOVE ITS AC3 TEMPERATURE AND BEFORE ANY SUBSTANTIAL GRAIN GROWTH OCCURS, REDUCING THE CROSS SECTIONAL AREA THEREOF SUFFICIENTLY TO DEFORM SUBSTANTIALLY ALL OF THE GRAINS THEREOF RAPIDLY COOLING TO BELOW THE TRANSFORMATON RANGE OF THE STEEL AND REPEATING SUCH HEATING, REPEATING SAID WORKING AND COOLING A PLURALITY OF TIMES TO PRODUCE AN ULTRAFINE GRAIN SIZE AND FOLLOWING THE LAST WORKING, QUICKLY COOLING TO SUBSTANTIALLY ROOM TEMPERATURE. 